Vehicle braking systems typically include a valve assembly known as a modulator which is connected to a source of pressurised fluid, the modulator being used to regulate supply of pressurised fluid to and from a fluid pressure operated brake actuator. The modulator has a supply inlet which is connected to the source of pressurised fluid, a delivery port which is connected to the brake actuator and an exhaust outlet which is connected to the atmosphere (or any other low pressure volume), and can adopt a build position in which flow of fluid between the supply inlet and the delivery port is permitted, an exhaust position in which flow of fluid between the delivery outlet and the exhaust outlet is permitted, and a hold position in which flow of fluid between any two of the exhaust inlet, delivery port and exhaust outlet is substantially prevented.
In conventional braking systems, control of the modulator is achieved using a pressurised fluid signal known as the braking demand signal. When there is driver demand for braking, the driver typically operates a foot pedal, and movement of the foot pedal generates a fluid signal which is transmitted to a control inlet of the modulator. Receipt of the braking demand signal causes the modulator to move to the build position, so that the supply of pressurised fluid from the source of pressurised fluid to the brake actuator required to operate the vehicle brake commences. When the fluid pressure in the brake actuator exceeds a predetermined level relative to the pressure of the braking demand signal, the modulator moves to the hold or “lapped” position. Finally, when the driver releases the brake pedal, there is no longer demand for braking, the braking demand signal is removed, and the modulator reverts to the exhaust position, so that the pressurised fluid in the brake actuator acting to apply the vehicle brake is exhausted to the atmosphere.
If the vehicle is provided with anti-lock braking, the braking system includes at least one electrically operable valve which can override the braking demand signal. This is controlled using an electronic braking control unit (ECU) in accordance with conventional ABS control algorithms momentarily to release the brake pressure by moving the modulator to the exhaust position, or hold the brake pressure by moving the modulator to the hold position, even if there is braking demand, if wheel lock is detected.
In electronic braking systems, the braking system is provided with electrically operable hold and exhaust valves. Operation of the foot pedal generates an electrical braking demand signal, and this is transmitted to the ECU, which operates the hold valve and exhaust valve to control the modulator to build, hold or release the pressure in the brake actuator as described above. In this case, supply of fluid to the control inlet is also from the supply of pressurised fluid.
An example of a prior art modulator 10 for use in a vehicle with an electronic braking system is shown in FIG. 1. The modulator 10 has a generally cylindrical housing having a supply inlet 12 which is adapted to be connected to a compressed air reservoir (not shown), a delivery port 14 which is adapted to be connected to a brake actuator (not shown), and an exhaust outlet 16 which in this example vents to atmosphere. It will be appreciated that the exhaust outlet 16 need not vent to atmosphere, and may instead be connected to an alternative low pressure volume which may be elsewhere in the vehicle braking system.
There is also shown a hold valve 18 which has an inlet 18a which is connected to the supply inlet 12 of the modulator 10, and an outlet 18b which is connected to a control chamber 22 of the modulator 10, and an exhaust valve 20 which has an inlet 20a which is connected to the control chamber 22 and an outlet 20b which vents to the atmosphere. The hold valve 18 has a valve member 18c which is movable from an open position in which flow of fluid between the supply inlet 12 and the control chamber 22 is permitted, and a closed position in which flow of fluid between the control chamber 22 and the supply inlet 12 is substantially prevented. Similarly, the exhaust valve 20 has a valve member 20c which is movable between an open position in which venting of fluid from the control chamber 22 to the atmosphere is permitted, and a closed position in which flow of fluid from the control chamber 22 to atmosphere is substantially prevented. Typically each of the valve members 18c, 20c is moved between the open and closed positions using electrical actuation means such as a solenoid or a piezoelectric element.
The control chamber 22 is located in the space between the modulator housing 24 and a first piston 26, hereinafter referred to as the control piston 26, which is movable within the housing 24 to vary the volume of the control chamber 22. A generally circular seal, which in this example is an O-ring 28 is provided in a circumferential grove around the control piston 26, and engages with the housing 24 to provide a substantially fluid tight seal between the housing 24 and the piston 26. It will be appreciated that the seal need not be an O-ring, and instead of having a generally circular cross-section, maybe X or Z—shaped in cross-section, or may comprise a lip seal or any other suitable sealing means which allows movement of the piston 26 relative to the housing 24 whilst providing a seal between the two parts.
The space within the housing 24 on the opposite side of the control piston 26 to the control chamber 22 is hereinafter referred to as the main chamber 30.
The supply inlet 12, delivery port 14 and exhaust outlet 16 each comprise an aperture in the housing 24 which is located on the opposite side of the first piston 26 to the control chamber 22, so that each enters the main chamber 30 of the modulator. A second piston 32, hereinafter referred to as the main piston 32, is provided in the main chamber 30.
The main piston 32 is provided with a central aperture 32a which extends through the main piston 32 from the side of the piston 32 adjacent the control piston 26 to the other. The area around this central aperture 32a provides a valve seat 32b, hereinafter referred to as the exhaust seat 32b. The face of the control piston 26 which forms an edge of the main chamber 30 is provided with a generally circular ridge 26a which has a larger diameter than the central aperture 32a in the main piston 32. The control piston 26 is movable in the housing 24 until the ridge 26a engages with the exhaust seat 32b, thus closing the central aperture 32a in the main piston 32.
A portion of the housing 24 forming the edges of the main chamber 30 is provided with a ledge 34 which extends around the entire housing 24 into the first sub-chamber 30a and which is provided with a generally circular ridge 34a which extends towards the main piston 32. The main piston 32 is movable into engagement with this ridge 34a, the portion of the main piston 32 engaging with the ridge 34a so as to substantially prevent flow of fluid between the main piston 32 and the housing 24, thus providing a valve seat hereinafter referred to as the reservoir seat 32c. 
A spring 44 is provided which acts on the main piston 32, pushing the reservoir seat 32c against the ledge 34.
When the control piston 26 is engaged with the exhaust seat 32b, and the main piston 32 is engaged with the reservoir seat 34a, an annular chamber 30a is formed in the main chamber 30 between the control piston 26 and the main piston 32, and the delivery port 14 is arranged to communicate with the chamber 30a. In other words, the main piston 32 divides the main chamber 30 into two chambers—the annular chamber 30a into which the delivery port 14 opens and a further chamber 30b into which the supply inlet 12 opens.
In this further chamber 30b is provided a separator 36 which has a generally cylindrical portion 36a which extends from the housing 24 around the exhaust outlet 16 towards the central aperture 32a of the main piston 32. The internal diameter of the cylindrical portion 36a of the separator 36 is similar to the diameter of the central aperture 32a of the main piston 32, and an O-ring 38 is provided between the cylindrical portion 36a of the separator 36 and the main piston 32. The O-ring 38 provides a substantially fluid tight seal between the separator 36 and the main piston 32 which acts to ensure that flow of fluid into the exhaust can only occur via the central aperture 32b in the main piston 32, whilst allowing movement of the main piston 32 into and out of engagement with the ledge 34.
The supply inlet 12 opens into the volume surrounding the cylindrical portion of the separator 36.
Flow of fluid between these apertures is controlled by movement of the control piston 26 and the main piston 32 as follows.
When there is no braking demand, the hold valve 18 is in the closed position, and the exhaust valve 20 is in the open position. The control chamber 22 is therefore vented to atmosphere, and the control piston 26 is located such that the volume of the control chamber 22 is minimum. The reservoir seat 32c is engaged with the ledge 34 so that flow of fluid from the supply inlet 12 to the delivery port 14 is prevented, and the exhaust seat 32b is out of engagement with the control piston 26 so that flow of fluid from the delivery port 14 to the exhaust outlet 16 via the central aperture 32a in the main piston 32 is permitted. The delivery port 14, and hence the brake actuator, is therefore vented to atmosphere, and no pressure is being applied at the brake.
When a braking demand signal is received, the hold valve 18 is operated so that the valve member 18c moves to the open position, and the exhaust valve 20 is operated so that the valve member 20c moves to the closed position.
The control chamber 22 is therefore no longer venting to atmosphere, and flow of fluid from the reservoir into the control chamber 22 causes fluid pressure in the control chamber 22 to increase. The control piston 26 is acted on by this increasing pressure and moves towards the main piston to increase the volume of the control chamber 22. As the control piston moves, it comes into engagement with the exhaust seat 32b on the main piston 32. At this point, the delivery port 14 is no longer connected to the exhaust outlet 16. As the fluid pressure in the control chamber 22 continues to increase, when it reaches a certain point, the force of the fluid pressure in the control chamber 22 acting on the control piston 26 is sufficiently large that the control piston 26 carries on moving to increase the volume of the control chamber 22 and therefore pushes the main piston 32 against the biasing force of the spring 44 so that the reservoir seat 32c moves out of engagement with the ledge 34. At this point, the supply inlet 12 can communicate with the delivery port 14, and flow of fluid from the reservoir to the brake actuator commences. The modulator 10 is said to be in the “build configuration”.
When the pressure in the brake actuator reaches the required level, and it is desired to hold that pressure, the hold valve 18 is operated to move the valve member 18c to the closed position. The control chamber 22 is therefore closed. As fluid continues to flow from the reservoir and into the modulator 10 via the supply inlet, fluid pressure in the main chamber 30 builds and acts on the control piston 26 against the pressure in the control chamber 22 to cause the control piston 26 to move back to reduce the volume of the control chamber 22. The main piston 32 may then move under the biasing force of the spring until the reservoir seat 32c comes into engagement with the ledge 34. At this point, flow of fluid from the reservoir to the brake actuator is prevented whilst the exhaust outlet remains closed, and the modulator 10 reaches an equilibrium at which the fluid pressure in all parts of the modulator 10 remains constant, and is said to be in the “hold configuration” or “lapped configuration”.
To release the brake pressure, the exhaust valve 20 is operated to move the valve member 20c to the open position. The fluid in the control chamber 22 is vented to atmosphere, and the fluid pressure in the main chamber 30 of the modulator 10 acts on the control piston 26, pushing it out of engagement with the exhaust seat 32b. Fluid may then flow from the brake actuator into the delivery port 14, through the central aperture 32a of the main piston 32 and be vented to atmosphere via the exhaust outlet 16. The modulator thus returns to the “exhaust configuration”.
It will be appreciated that when the hold valve 18 and the exhaust valve 20 are first operated to bring the modulator 10 from the exhaust configuration to the build configuration, movement of the main piston 32 to open the reservoir seat 32c will not commence until the pressure in the control chamber 22 is such that the force acting on the control piston 26 is sufficient to overcome the frictional forces between the control piston 26 and the housing 24, frictional forces between the main piston 32 and the separator 36, and, most significantly, the “energisation force” of the reservoir seat 32c. The “energisation force” of the reservoir seat 32c is the force pushing the main piston 32 against the ledge 34, and it has been found that to provide adequate sealing to substantially prevent leakage of fluid across the reservoir seat 32c an energisation force of the order of 1 N per millimeter of seat circumference is required.
In this case, the energisation of the reservoir seat 32c constitutes a combination of the force resulting from the fluid pressure at the supply inlet 12, i.e. the reservoir pressure, and the biasing force of the spring 44, and the area of the main piston 32 subject to the reservoir pressure and spring force can be tailored to provide the desired 1 N/mm energisation. For a standard sized modulator, in which the control piston 26 is around 92 mm in diameter, the force required to move the modulator 10 from the exhaust configuration to the build configuration is around 100N, which translates to a pressure in the control chamber 22 (referred to as “the cracking pressure”) of around 0.25 bar. A cracking pressure of this level is generally considered to be acceptable—significantly higher cracking pressures would give rise to an unacceptable delay between initiation of the braking demand signal and the start of brake actuation.
A problem arises if this prior art configuration of modulator 10 is reduced in size—for example because it is desired to provide a modulator at each wheel, rather than one central modulator which operates a plurality of brake actuators. This is because the cracking pressure is inversely proportional to the square of the diameter of the control piston 26, and, whilst the reduction in modulator dimensions will result in a reduction in the total energisation and frictional forces, both these forces are proportional to the main piston diameter (not to its square). For example, if the diameter of the control piston 26 is reduced from 92 mm to 50 mm, with this configuration of modulator, the cracking pressure increases to around 1 bar. This is too high to be generally acceptable.
The disadvantages of the prior art are overcome by the present invention, and an improved valve assembly is hereinafter disclosed.